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Optics

Optics. Kenneth Nordsieck University of Wisconsin. Refractive Optics - Design rationale Performance Slitmasks Gratings Polarimetric Optics Risks and Concerns. Design Goals. wavelength range 320 – 900 nm preserve 850 nm – 1.7 m future beam all transmission; spherical surfaces

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Optics

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  1. Optics Kenneth Nordsieck University of Wisconsin • Refractive Optics - Design rationale • Performance • Slitmasks • Gratings • Polarimetric Optics • Risks and Concerns SALT PFIS PDR - Optics

  2. Design Goals • wavelength range 320 – 900 nm • preserve 850 nm – 1.7m future beam • all transmission; spherical surfaces • beam size 150 mm • images 0.3 arcsec (dispersion direction) • maximum use of SALT 8 arcmin FOV SALT PFIS PDR - Optics

  3. Detector/Camera Design • Detector size • camera F > 2.2 => spatial 60 mm • spectral res elements => dispersion 90 mm • 15m pixels 0.13 arcsec • Camera • speed, wavelength range => NaCl • FOV 18 deg drives complexity • filters near detector for affordable size • redesigns have eliminated 4 elements and 1 group SALT PFIS PDR - Optics

  4. Camera Layout • NaCl is center element in 2 triplets • Uses symmetry principle to reduce aberrations SALT PFIS PDR - Optics

  5. Beam/Collimator Design • Beam • Diameter 150 mm set by maximum etalon size • Pupil at VPH grating, between etalons • Shutter (modified Prontor E150) just before etalons • Beam length set by NIR beam fold mirror • Collimator • Telescope F/4.2 8' (105 mm). Flat. • Entrance pupil distance 60 cm - need strong field lens • Space for waveplates after field lens • Last element after Visible NIR split: different for each beam • Redesign eliminated 2 elements (including 1 NaCl). SALT PFIS PDR - Optics

  6. Collimator Layout • 1 NaCl triplet • Focus is at final doublet SALT PFIS PDR - Optics

  7. Coatings • Baseline: MgF2/ SolGel • Best broad band performance • Durability concerns • Collimator • needs very broad band for NIR • Fallback MgF2 • Camera • Fallback Goodman multilayer • Fallback all surfaces: down 20% • FPRD minimum: down 10% • Fold Mirror LLNL coating SALT PFIS PDR - Optics

  8. Imaging Performance • Imaging specification • EE50 < 0.3 arcsec enslitted energy • EE50 < 0.4 arcsec encircled energy • Well within spec, allowing for manufacturing and assembly errors SALT PFIS PDR - Optics

  9. Volume Phase Holographic Gratings • Grating parameters • Groove density n (600 – 3600 l/mm) • Hologram thickness D (> 2m) • index modulation Dn (< 0.1) • tilt a (6 – 45 deg): R = 1000-6500 • Approximate properties (m = 1, “Kogelnik” approximation) • blaze peak lB = l (b=a) • superblaze peak ~ Dn D • bandwidth Db ~ 1/nD • Procedure • choose n to select l(R) • maximize Db for a given superblaze peak: • choose maximum Dn = 0.1 • choose D to place superblaze peak SALT PFIS PDR - Optics

  10. Grating Complement • 780 l/mm VPH: R = 1500, 2x wavelength coverage blue and red (on axis) (non-Kogelnik behavior) • 300 l/mm conventional transmission grating: R = 500, 2x wavelength coverage to 3 arcmin off axis • 4 high-density VPH’s: R = 2500 - 6500 SALT PFIS PDR - Optics

  11. On-axis Coverage VPH Field of View • FOV perpendicular to dispersion: full 8' • Useful field of view parallel to dispersion is limited for VPH by blaze shift • For PFIS gratings: efficiency > 50% over 4' parallel to dispersion; usable to 6'. • Possible improvement: if Dn > 0.1 proves feasible, can reduce D and increase bandwidth (30%?) • For low dispersion programs needing largest possible FOV, may be advantageous to provide conventional 600 l/mm transmission grating SALT PFIS PDR - Optics

  12. Overall Optics Efficiency • For VPH, R = 4000 • Spectrograph/ Detector efficiency near 60% 400 - 800 nm • FPRD Minimum ~ 40%, including CCD, coatings, VPH conservatism (about 10% each) • On Sky efficiency >20% (relative to full unobscured centered 11m aperture) SALT PFIS PDR - Optics

  13. Polarimetric Optics • SALT Telescope instrumental polarization • Primary: Al • SAC: LLNL • < 0.1% 4 ' dia FOV • ~0.2% at 8' dia • Field effect > track effect • FPRD: correctable to < 0.04% SALT PFIS PDR - Optics

  14. Polarimetry - Beamsplitter • Wollaston Beamsplitter in collimated beam after grating • Split +/- 45 deg polarizations ~ 5 deg => 4 arcmin at detector into two half-fields “O” and “E” • Mosaic of 9 calcite prisms in framework • Chromatic dependence of splitting: 20 arcsec from 320 – 900 nm SALT PFIS PDR - Optics

  15. Polarimetry - Waveplates • Pancharatnam superachromatic waveplates: stack of 6 very thin retarders • In collimator after field lens (to minimize diameter) • ½ and ¼ waves from 320 – 1.7 microns • very large SALT etendue (aperture x FOV) limits performance of waveplates in UV – reduced efficiency; sensitivity to pupil SALT PFIS PDR - Optics

  16. Waveplate efficiency • Pancharatnam modified for off-axis performance • Overall polarimetric efficiency reduced, but still > 98% (halfwave), 94% (quarterwave) • Pupil shape sensitivity not significant for halfwave • Quarterwave more sensitive to pupil effects, due to manufacturing limits on element thickness SALT PFIS PDR - Optics

  17. Slitmasks • Adopt Gemini GMOS slitmask technique • observation unique carbon-fiber slitmasks, cut by dedicated commercial laser milling machine off-site • inventory of ~60 invar frame mask holders • estimate ~$1.50/ carbon fiber mask, < 1 hour to mill • Magazine • removable to be loaded off-telescope • 30 carbon-fiber slitmasks; 10 aluminized longslits • bar-code identifier SALT PFIS PDR - Optics

  18. Slitviewing Optics • Estimate ~50% of programs are longslit: tilted aluminized slitjaws would provide direct view of slit for acquisition/ guiding • Longslits handled/ stored by same mechanism as carbon-fiber slitmasks (double-thick) • Proposed slitviewing optics using all spherical mirrors (Offner relay), feeding existing acquisition camera • Not yet approved by project (difficult integration with prime Focus Platform) SALT PFIS PDR - Optics

  19. Procurement Status • Refractive optics • Lick Obs D. Hilyard $266,830 14 months • Janos • Crystran C. Wallace Quote in work • Specac A. Afran Quote in work • Polarimetric optics - beamsplitter • Karl Lambrecht V. Vats $44,100 30 wks • Waveplates • Karl Lambrecht V. Vats $120,000 8-10 mo 4x4 mosaic • Bernhard Halle Will quote on 1/2 wave 2x2 mosaic SALT PFIS PDR - Optics

  20. Risk Status • Refractive optics - Low to medium • complexity reduced • number of groups comparable with similar optics • quotations being obtained from 4 vendors • cost and schedule comparable to Concept Proposal • Polarimetric optics - medium • quotations being obtained from 2 vendors • beamsplitter cost similar to Concept Proposal • waveplates. Lambrecht likely to drop out. Halle experience with similar waveplates on VLT/FORS1 • if necessary mitigate cost with descope of FOV to 4 arcmin; use largest Halle monolithic waveplate (60 mm). Put cost cap $160K total on waveplates and trade among FOV and configurations SALT PFIS PDR - Optics

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